Protein function assignment has been benefited from genetic methods, such as target gene disruption, RNA interference, and genome editing technologies, which selectively disrupt the expression of proteins in native biological systems. Chemical probes offer a complementary way to perturb proteins that have the advantages of producing graded (dose-dependent) gain- (agonism) or loss- (antagonism) of-function effects that are introduced acutely and reversibly in cells and organisms. Small molecules present an alternative method to selectively modulate proteins and to serve as leads for the development of novel therapeutics.